The convenience of transporting and storing industrial gases such as oxygen and nitrogen including LNG is well established. At the use site, the liquefied gas is stored in liquid form at a cryogenic temperature in the range of about −100° F. to −320° F. The liquefied gas is then vaporized and superheated to near ambient temperatures before use. Various heat sources are used to supply the heat for vaporization such as waste process heat, seawater, fired heaters and ambient air. For example, in the case of LNG which is used as a fuel gas, about 2% of the combustion heat of the fuel gas is required for vaporization of the LNG and, for this reason, ambient atmospheric air is a desirable heat source. In Patent Publication U.S. 2010/0043452 A1, Baudat uses water or other intermediate heat transfer fluid loop in which the water or heat transfer fluid is heated as the water is recirculated through an air tower. When the air is too cold, supplemental heat is provided to the process.
The disadvantage of using an intermediate heat transfer fluid loop in any heat transfer process, such as the water loop of Baudat, is that there is a temperature difference loss within each separate fluid (water) loop. These temperature differences are additive that reduce the useful range of the ambient air temperature which may be utilized economically in a process such as Baudat, notwithstanding the added complexity and cost of the apparatus.
In Patent Publication 2007/0214805 A1, Armstrong et al describes a shipboard LNG vaporizer using ambient air and an intermediate heat transfer fluid together with redundant vaporizers to allow for defrost cycles.
In Patent Publication 2010/02505979 A1, Gentry et al describes a heated fluid LNG regasification apparatus.
In Patent Publication 2003/0159800 A1, Nierenberg uses seawater as the heat source for LNG regasification.
In Patent Publication 2007/0214806 A1, Faka continuously regasifies LNG using ambient air together with an intermediate fluid heat transfer loop auxiliary heater wherein the ambient air heater is subjected to a defrost cycle.
In Patent Publication 2011/0030391 A1, Faka employs a mechanical device to remove frost from his continuous ambient air vaporizer and additionally adds an intermediate heat transfer fluid loop.
In Patent Publication 2010/0101240 A1, Mak describes a forced ambient air vaporizer wherein the moist air is dehydrated for subsequent use within his multi-chambered vaporizer system. A temperature control scheme maintains Mak's air above 32° F.; however, no instructions of fin surface temperature where ice may form is discussed in Mak's complicated and costly apparatus.
In Patent Publication 2009/0126372 A1, Faka describes a forced ambient air continuous regasifier that employs a source of heat to intermittently defrost his vaporizer.
Vogler, Jr. et al in U.S. Pat. No. 4,399,660 (1983) describe an ambient air vaporizer with a particularly wide space between their finned tube vaporizer elements to allow for ice growth therein. A steady state frost/ice layer is claimed.
At the AIChE May 2000 spring meeting, Paper #58e, PP 188-196, Bernert further discusses this ice growth problem.
In U.S. Pat. No. 3,293,871 (1966), Tyree, Jr. attaches fins to his vaporizer tubes, said fins being in thermal contact with the tube by suitable means such as soldering. A fan is used to provide a constant stream of ambient air across the fins. Tyree states that although he has ice growth, “it is highly unlikely” for the heat transfer surface to become iced over thus providing defrost means.
In U.S. Pat. No. 5,390,500 (1995), White et al describes various means to manage the ice growth common to ambient air vaporizers. Various concentric tubular assemblies are postulated which rely on flowing or stagnant gas layers combined with internally finned elements partially filled with various filler materials that are in contact with the fluid to be vaporized. It is well known that apparatus used for certain cryogenic liquefied gases such as liquid oxygen should avoid direct contact with such materials. A multiple tube combination is described to complete the apparatus.
In U.S. Pat. No. 3,124,940 (1964), Guelton describes a mechanical defrosting device for a Fan-Ambient air vaporizer thus illustrating an early awareness of the frost/ice formation problem associated with ambient air cryogenic vaporizers.
Booth, in U.S. Pat. No. 2,322,341 (1943), describes an extruded axially-finned aluminum heat exchange element for refrigerants to be evaporated. Such elements are presently used in many different embodiments in cryogenic vaporizers.
In U.S. Pat. No. 3,735,465 (1973), Tibbetts et al describes a finned tube assembly for use in cryogenic vaporizers wherein the extended surface portions are clamped or locked directly onto an elongated tubular member such that “complete contact” is made between the surface to achieve “optimum heat transfer characteristics” and thus “minimizing the thermal contact resistance between the tubing and the hub”. When assembled into a multi-element vaporizer, a fan may be employed. Conversely rather than “minimizing the thermal resistance” of Tibbetts et al, the invention of the present application, as described and claimed, purposely introduces a particular thermal resistance to heat transfer to achieve improved performance.
Similarly to Tibbetts et al, Lutjens et al in U.S. Pat. No. 4,487,256 (1984) describes a clamped fin tube assembly for cryogenic ambient air vaporizers which describes less frosting in the hub area and further that the tube is in intimate contact with the outer sleeve halves to form a common forced ambient air cryogenic vaporizer heat exchange element. Mentioned also is the use of a “thin coating (0.001 inch-0.100 inch) of fluorocarbon or Teflon applied to the” internal cylindrical surface of the hub such that the layer is so thin that even only a temperature drop of 1° or (so) has been encountered across this film, which statement indicates a failure of the prior art to appreciate the nature of the frost growth problem in these vaporizers.
In Patent Application 2007/0214807 A1, Faka employs ambient air with an air heater to prevent icing in similar fashion as Katare does in Patent Application 2007/0250795 A1. In U.S. Pat. No. 8,069,678 B1 (2011), Bernert describes an improved regasification ambient air heat exchange element employing a thermally conductive adhesive to bond the inner fluid tubular conduit to the outer finned hollow bore heat transfer element to improve heat transfer between the ambient air and the cryogenic fluid again, as in Tibbetts above, accepting ice growth as a given to be accommodated with alternate design features.
The reason why atmospheric vaporizers are not used more widely for continuous service is because ice and frost build up on the outside surfaces of the vaporizer that are exposed to the moist ambient air. Not only does the ice inhibit effective vaporizer capacity, the weight of the ice creates a structural problem as well as requiring greater space or larger sized units (for example, in Vogler, Jr. described above) to accomplish a given rate of regasification. Where continuous operation is required, either auxiliary heat or switching redundant modules have been shown in prior art to be necessary.
For the foregoing reasons, there remains a need for a process and apparatus for regasifying or vaporizing cryogenic fluids using only ambient air in direct contact with the cryogenic heat exchange elements which apparatus permits ice-free/frost-free continuous operation of cryogenic fluid vaporizers that use only ambient air as the heat source.